Integrated circuits (ICs) having a plurality of semiconductor devices including field effect transistors (FETs) are a cornerstone of modern microelectronic systems. Conventionally, the various regions of FETs (e.g., source/drain and source/drain extensions) are formed by introducing dopant atoms into a semiconductor substrate using methods such as ion implantation, etc. After the dopants have been introduced, they are electrically activated by subjecting the semiconductor substrate to one or more annealing processes such as low temperature thermal annealing, rapid thermal annealing, spike annealing, flash annealing or laser annealing.
Dopants, however, have a tendency to diffuse or expand both laterally and vertically away from the profile during annealing thereby increasing the dimensions of the various device regions. This diffusion of dopants is undesirable particularly as semiconductor devices are scaled down in size. Scaling device dimensions down to the molecular regime thus presents a fundamental and technological challenge for fabricating well defined structures with controlled atomic composition.
One proposed route for achieving fine control of structural composition is the integration of self-limiting and self-assembly processes where surface and chemical phenomena guide the synthesis and fabrication of the desired nanostructures. There is a need for technology to demonstrate reliable nanoscale doping of silicon structures, e.g., for well-defined and uniformly doped ultra-shallow junctions at the source and drain extension regions. Conventional ion implantation processes which rely on bombardment of semiconductors with energetic ions suffer from an inability to achieve an implantation range and abruptness down to the nanometer range, a stochastic spatial distribution of the implanted ions, an incompatibility with nanostructured materials, and crystal damage. Solid-source diffusion processes lack the desired uniformity and control over the areal dose of the dopants to be used for miniaturized device fabrication. Monolayer doping (MLD), however, can attain a controlled doping of semiconductor materials with atomic accuracy. Generally, MLD utilizes the crystalline nature of semiconductors to form highly uniform, self-assembled, covalently bonded dopant-containing monolayers followed by a subsequent annealing step for the incorporation and diffusion of dopants.
Exemplary monolayer formation reactions are self-limiting and result in a deterministic coverage of dopant atoms on the semiconductor surface. MLD differs from other conventional doping techniques by method of dopant dose control. For example, as compared to ion-implantation, MLD does not involve a highly energetic introduction of dopant species into the semiconductor lattice where crystal damages are induced. To prevent dopant loss, however, conventional MLD techniques require an oxide cap layer to protect respective dopants during subsequent thermal processes. Thus, there is a need to provide or fabricate ultra-shallow junctions without depositing and/or removing such an oxide cap.